The invention relates to a method for operating a four-stroke internal combustion engine of the generic type specified in the pre-characterizing clause of Claim 1.
Internal combustion engines, in the compression ignition of homogeneous lean air/fuel mixtures, afford the possibility of low nitrogen oxide formation and high thermal efficiency. However, these advantages arise only within a narrow operating range which depends on a multiplicity of rapidly changing boundary conditions.
In compression ignition, the air/fuel mixture is brought to ignition by means of compression heat. After the start of ignition, a self-accelerating combustion process is initiated by the energy released at the same time. Compression which is too low leads to delayed incomplete combustion, and compression which is too high leads to inadmissibly steep pressure rises and to gas oscillations in the working space (knocking combustion).
DE-195 19 663 A1 describes a method for operating an internal combustion engine with compression ignition. Here, in a first step, a homogeneous and lean air/fuel mixture produced by external mixture formation is compressed nearly to the ignition limit. In a second step, an additional quantity of the same fuel is finely atomized and, avoiding contact with the wall, is injected into the working space. The fuel injected late forms a mixture cloud which ignites, since, because of the higher fuel content, its ignition limit is below the compression temperature reached in the first step.
The object on which the invention is based is to provide a method of the generic type specified in the pre-characterizing clause of Claim 1, by means of which low-consumption and low-pollutant combustion adapted to the respective operating range is possible.
The object is achieved by means of a method having the features of Claim 1.
The timing and profile of combustion are critical for low-consumption and low-pollutant operation free of misfires and knocking. A start of combustion in the region of top dead centre is optimal in terms of consumption and pollutants. By contrast, delayed combustion causes fuel consumption and pollutant emission to rise and leads to combustion misfires, whereas premature combustion leads to inadmissible pressure rises and peak pressures with knocking phenomena.
In an internal combustion engine, the temperature of the mixture is increased as a result of the geometric compression of the closed-off maximum initial volume to a remaining residual volume. A temperature which over time brings the mixture to the ignition threshold is established in the compressed volume. The combustion process which follows the compression ignition of the mixture is a process which is self-accelerating on account of the energy released as a result of combustion. During the combustion process, the reaction continues to accelerate. The high reaction rate of combustion leads to sharp pressure rises in the combustion space which constitute an undesirable form of combustion. The fuel quantity ready for ignition in the combustion space has an effect on the pressure profile, that is to say the greater this fuel quantity is, the sharper the rise in reactivity and the steepness of the pressure rise become. The limits of combustion in the case of the compression ignition of homogeneous lean mixtures towards a higher load, with consequently greater fuel quantities in the combustion space, is attributable to the fact that the temperature distribution in the air/fuel mixture in the combustion space is too homogeneous. The problem does not arise with small fuel quantities in the combustion space, since the released energy quantity is not so great that the continuous self-acceleration of the combustion process could lead to an undesirably steep pressure rise in the combustion space.
In the case of a homogeneous mixture distribution and compression, a large quantity of air/fuel mixture is brought near to ignition simultaneously, and many mixture fractions reach the auto-ignition condition at the same time. Inherent in the principle of self-acceleration is the activation of reactivity by means of a differential energy quantity, leading to the release of a multiple of the differential energy quantity released.
The delay in the ignitions of the mixture may delay the release of energy, so that limited pressure rises which are in an acceptable range occur in the combustion space. An ignition delay takes place as the result of an absorption of the combustion energy released. Since the ignition operation is an integral process in energy terms, a delay in energy release can be achieved by means of a time-related and local spreading of the auto-ignition operation. Instead of ignition being based on the homogeneity of the mixture with a similar tendency to ignition, it is now based on a gradient of the ignition performance of the mixture, thus leading to a time-delayed energy reaction in the combustion space, the pressure rises being lower because of delayed combustion. The gradient of the ignition performance in the mixture is found differentially in all the mixture fractions which are involved in the injection. The gradient should not be so great that the temperature gradient established in the mixture is such that the energy transmission of the combustion reaction is determined by heat conduction.
A time-related and local spreading of the ignition performance can be set by means of a different energy absorption over time and space. Energy is absorbed due to the evaporation and intermixing energy of the fuel in the air and to the specific heat capacity of the mixture during the auto-ignition phase. With the aid of internal mixture formation in the reciprocating-piston engine, it is possible to produce a local gradient of the ignition performance via the mixture volume. The fuel quantity is introduced into the medium present in the combustion space, in such a way that combustion-space regions with different fuel evaporation are obtained. In the regions with earliest and best evaporation, the fuel fractions receive the highest energy supply over time and, by virtue of this, form the greatest tendency to ignition. The regions where intermixing is still incomplete still absorb energy during evaporation, that is to say delay the tendency to auto-ignition. Energy absorption due to evaporation gives rise to the gradient of the ignition performance over the combustion space. For the loading increase during combustion with compression ignition, gradient formation is possible by the injection operation being stratified into a plurality of injection operations. Time-stratified injection consists, during compression, of already highly ignitable fuel regions. Thee fuel fractions freshly supplied to the combustion space absorb energy from the compressed mixture during their treatment phase and delay the ignition process of the already precompressed mixture. The energy absorption reduces the pressure rises.
For designing a reciprocating-piston engine with variable effective compression, for example by the free activation of the inlet and outlet members via the uniform variation in the combustion-space volume, mixture formation can be combined with the retention of exhaust gases in the combustion space in order to influence the next combustion operation. For exhaust-gas retention, in the case of a reduction in the combustion-space volume the outlet member is closed earlier, with the result that the exhaust-gas quantity is compressed and subsequently expanded over the minimum volume. In the case of early opening directly after the time of the minimum volume and of subsequent pressure equalization between the combustion space and surroundings, a uniform stratification of hot exhaust gas in relation to the cold fresh gas is established in the combustion space. During late opening of the inlet members, because of the greater pressure difference a higher inflow energy is provided which reduces the stratification of exhaust gas and fresh gas.
In two-layer combustion-space temperature distribution, the fuel can be injected early for low loads and the consequently small fuel quantity. The small fuel quantities are distributed in the fresh gas effectively and uniformly. The compact charge of exhaust gas, because of its low surface/volume ratio, acquires a high temperature which leads to the easy ignition of the fresh mixture with a high air excess. The high air excess prevents the acceleration of the reaction towards steep pressure rises. The introduction of the fuel into the fresh-charge region counteracts the unintended processes of fuel reaction, such as, for example, high-temperature combustion, which leads to the formation of NOx, or dehydration and coagulation, which yield soot. For higher loads, the opening of the inlet valve is delayed, thus leading to better intermixing of the hot exhaust gas and of the colder fresh air. For a homogeneous temperature distribution in the combustion space, late injection or the separation of the injection operation in the way illustrated above can give rise to a gradient in the ignition performance. Early injection during the turbulent mixture formation of the fresh gas with the exhaust-gas fractions in the combustion space as a result of the late opening of the inlet valve allows the fuel fraction injected earlier to be distributed homogeneously in the combustion space and to ignite as a result of a temperature increase over the treatment time. The second fuel fraction is supplied later, in order to bring about a local lowering of the ignition performance as a result of the absorption of energy due to evaporation.
One limit value is determined by the optimally uniform mixture treatment as a result of very early injection. The opposite limit value is formed by the introduction of the mixture in the combustion space so late that the evaporation of the fuel can no longer take place before the reaction of sufficiently large fractions of the treated fuel. The high energy states in the surroundings of the incompletely treated and unburnt fuel lead to dehydration operations and coagulations in rich ranges, up to the formation of soot in the combustion space and in the exhaust-gas tract. The method according to the invention is carried out exclusively between these two limits mentioned.
Solid-borne sound, ionic current and rotational non-uniformity are measurement quantities which, simultaneously with combustion, reproduce the position and profile of the latter and thereby afford the precondition for rapid control action. This control action is made possible, using stored characteristic maps or employing high-speed adaptive electronics on the principle of neural networks. In addition to the data which monitor combustion, these electronics also take into account the valve control times and the injection times and also the air excess values derived from them.